A tunnel boring machine (“TBM”) is a tunnel excavation apparatus that is used to bore through the solid earth or ground (e.g., soil, rock, strata, mixed media, etc.). An exemplary tunnel boring machine is disclosed in U.S. Pat. No. 9,010,872, to Lenaburg, which is hereby incorporated by reference in its entirety.
Typically, a TBM includes a rotating cutterhead having a plurality of cutter assemblies mounted to the cutterhead. The rotating TBM cutterhead is pressed with a large thrust force against the ground or surface to be bored. The cutter assemblies exert local forces on the rock face to induce fracture or other structural failure in the ground substrate. Loosed material is then conveyed away to progressively form the desired tunnel. A conventional TBM produces a smooth circular tunnel wall, typically with minimal collateral disturbance. Prior TBMs used sturdy grinding spikes mounted to the cutterhead. However, the spikes would frequently break, typically requiring costly and dangerous repairs in situ. A breakthrough development by James S. Robbins was the invention of disc cutter assemblies that are rotatably mounted on the cutterhead. Robbins discovered that by replacing grinding spikes with rotatable disc cutter assemblies the reliability and performance of TBMs was significantly improved. Modern TBMs typically use rotatable disc cutter assemblies mounted to the cutterhead. As the cutterhead is pressed against the boring surface and rotated, the rotatable disc cutter assemblies fracture, crush, and loosen materials in the ground, which are then transported away, typically using a conveyor system progressively constructed behind the TBM as it advances.
FIG. 1 is a cross-sectional view of a prior art cutter ring assembly 80 for a tunnel boring machine (TBM), as disclosed in U.S. Pat. No. 8,783,786, to Shanahan et al., which is hereby incorporated by reference in its entirety. The cutter ring assembly 80 includes a shaft 81 that is configured to be fixedly attached to the TBM rotating cutterhead (not shown). An annular cutter ring 82, also referred to as a cutter disc, is positioned on a hub 83 between a retainer ring 84 and a shoulder 83A formed in the hub 83, to form a ring assembly 85, also referred to as a disc assembly.
The ring assembly 85 is rotatably mounted to the shaft 81 with a pair of bearing assemblies, each bearing assembly comprising an inner bearing race 87 fixed to the shaft 81, an outer bearing race 88 fixed to the hub 83, and a plurality of tapered roller bearings 89 rotatably retained between the inner and outer bearing races 87, 88. Oppositely disposed end retainers 90, 91 are provided on either side of the bearing assemblies, and engage the shaft 81 to hold the cutter ring assembly 80 together. During operation, the ring assembly 85 is rotatable about the shaft 81, and the end retainers 90, 91 are fixed to the shaft 81.
A rotary seal group 92 is provided at the interface between each of the end retainers 90, 91 and the ring assembly 85. The rotary seal groups 92 in this assembly 80 are mechanical face seals, also referred to as duo cone seals. The mechanical face seals were developed for protecting equipment working in the most adverse conditions, and comprise a pair of annular metal seal rings 93 and a pair of elastic toric members 94 (e.g., O-rings). The outer metal seal ring 93 engages the associated end retainer 90 or 91 through one toric member 94 and is fixed. The associated inner metal seal ring 93 engages the ring assembly 85 through a second toric member 94 and rotates. The two associated metal seal rings 93 abut to form a moving seal interface. Typically the available interior volume between the end retainers 90, 91 is filled with a lubricant, e.g., oil or grease. The rotary seal groups 92 provide a seal to retain the lubricant and prevent the incursion of dirt that could damage or destroy the bearing assemblies.
During tunnel boring operations the cutter ring 82 is urged against a rock face with sufficient force to crush and then produce fracturing stresses in the rock face. It is desirable that the width of the radially outer surface of the cutter ring 82 be small in order to reduce the contact area and increase the stress concentration produced on the rock face. However, the cutter ring 82 must have sufficient strength to maintain structural integrity under the extreme load conditions typical for TBMs.
It is also important for the ring assembly 85 to rotate during boring operations. The ring assembly 85 relies on frictional forces between the cutter ring 82 and the rock face to rotate the ring assembly 85 during boring operations, (i.e., while the cutterhead is pressed against the rock face and rotated about its own axis). If the cutter ring 82 stops rotating during boring operations, or slows sufficiently such that the cutter ring 82 skids along the rock surface, the cutter ring 82 will wear preferentially in the region engaging the rock face. Moreover, if the cutter ring 82 begins to preferentially wear at a particular location, the worn region will tend to subsequently stall at the worn location, resulting in growth of the worn region, which can eventually result in the effective loss of the cutter ring assembly 80.
TBMs are increasingly used to tunnel in regions that include relatively soft ground conditions. TBMs configured to operate in such conditions are sometimes referred to as earth pressure balance (EPB) machines. In these regions there may be insufficient frictional contact to ensure that the cutter ring 82 rotates on the shaft. Improvements in the cutter ring assembly are needed to avoid or mitigate the loss of cutter assemblies during operation of the tunnel boring machine. It is very expensive to stop TBM operations to replace or repair cutter assemblies.